U.S. Pat. No. 5,121,426 discloses a teleconferencing unit with an upwardly aimed loudspeaker and multiple gradient microphones arranged evenly around the loudspeaker. Each microphone has a polar response pattern with a major lobe. The loudspeaker is located in a null adjacent to the major lobe of each microphone. This reduces acoustic coupling between the loudspeaker and the respective gradient microphones. In one embodiment, the speakerphone has four first-order gradient microphones each having a supercardioid polar response pattern. The nulls are aimed at 125° with respect to the main lobe directions. In another embodiment, the speakerphone has six first-order gradient microphones pairwise electrically connected to form three reversible second-order gradient microphones, each having nulls at 90° and 180°.
The first- and second-order gradient microphones disclosed in the above patent are relatively expensive to manufacture, which makes the disclosed teleconferencing unit relatively expensive as well. In addition, the achievable reduction of acoustic coupling between the loudspeaker and the gradient microphones is limited due to manufacturing tolerances and changing acoustic behavior of the room. Furthermore, the optimum shape of the disclosed teleconferencing unit depends on the desired directional characteristics of the microphones. Also, the disclosed microphones have a relatively low signal-to-noise ratio (SNR) at lower frequencies.